This invention relates to method and apparatus for testing pairs of conductors of a telephone cable and the like, and to method and apparatus for measuring capacitance of circuit components such as conductor pairs. The invention is especially adapted for use in testing telephone cable splices.
A typical telephone cable is divided into groups of twenty-five pairs of conductors, usually with splicing, testing and the like being performed on twenty-five pairs at a time. Various abnormal conditions may exist in the cable such as shorts, splits, hot lines, open lines, crosses, and grounds. One mode of determining the existence of such faults in the past has been to measure the capacitance of each of the twenty-five pairs, arbitrarily select one of the pairs as a standard or reference, and compare the measured capacitance of each pair with this arbitrarily selected reference pair. If the difference exceeds a predetermined limit, then the pair outside the limit is considered to have a fault and further testing is performed to identify the type and location of the fault. However there is an inherent problem in this mode of testing which arises when the arbitrarily selected reference pair itself has a fault or is at one extreme or the other of the acceptable condition. Then erroneous fault indications are obtained and faulty pairs are not identified. This type of testing cannot be accomplished by utilizing a specific capacitance value selected in advance, because the normal capacitance value for a conductor pair of a cable varies with the length and installation conditions of the particular cable. The actual length of the cable usually is not known.
It has been suggested that in lieu of arbitrarily selecting a pair as a reference for the capacitance comparison, that the average value of all the measured capacitance values be utilized. However this approach is not satisfactory because the average value will change as a function of the number of faults present in the cable, again leading to erroneous indications.
Accordingly, it is an object of the present invention to provide new and improved method and apparatus for testing a telephone cable or the like having a plurality of pairs of conductors. A further object is to provide such method and apparatus wherein the capacitance value of each pair of conductors is measured, after which the median value of capacitance is selected and used as the reference value for comparison with each of the measured values, with pairs having capacitance values outside predetermined limits being indicated as having a fault.
It is another object of the invention to provide new and improved method and apparatus for measuring the capacitance of a circuit component such as a conductor pair. An additional object is to provide such method and apparatus utilizing the circuit component in the timing circuit of a multivibrator for controlling an oscillator output, with the oscillator output being counted resulting in a digital value which is directly related to capacitance. An additional object is to provide such method and apparatus wherein the circuit component is charged and discharged, with the elapsed time for charge or discharge being measured to provide the digital output indicative of capacitance.
Other objects, advantages, features and results will more fully appear in the course of the following description.